The field of this invention is millimeter and submillimeter waves and more particularly it relates to a tunable oscillator in this frequency range which is electronically tuned and thus requires no moving parts. The device utilizes a Gunn diode in conjunction with a dielectric image waveguide as the oscillator. The Gunn diode can be arranged to function as both an oscillator and as a mixer and thus a self-oscillating mixer with electronic tuning can be easily constructed using the teachings of this invention. Oscillator tuning is accomplished by magnetizing a region of the dielectric waveguide in the vicinity of the Gunn diode.